Method and devices to limit a creep of mechanical fasteners

ABSTRACT

A method and devices to limit or to exclude operating creep of mechanical fasteners is disclosed. The method consists in use of mechanical fasteners such as bolts, gaskets, screws, washers and other power elements manufactured from shape memory alloys having temperature interval of martensitic phase transformations corresponding to the operating temperature of industrial equipment. The power elements are previously shape-memorized either to tension or to compression, flexion, torsion, or to their combinations under temperature of martensite state with suitable quantity of conserved residual shape-memorized deformation obtained during loading-unloading of the power elements. Further constrained recovery of shape-memorized deformation under operating temperature generates reactive shape-recovering forces having direction inverse to the direction of operating creep. The process of creep limitation is called “Method of “negative creep”.

CROSS-REFERENCE TO RELATED APPLICATION

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FEDERALLY SPONSORED RESEARCH

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SEQUENCE LISTING OR PROGRAM

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BACKGROUND OF THE INVENTION—FIELD OF INVENTION

This invention relates to mechanical fasteners, specifically to bolted flanged connections with gaskets that have to provide a tight and durable joint between component parts of pressure vessels, piping systems, and other industrial facilities under external and/or internal loadings and operating elevated temperatures.

BACKGROUND OF THE INVENTION

One of the most typically used means to obtain the tight and durable joint between component parts of industrial facilities is to connect their pieces with bolted fasteners. These connections have a wide applicability in petrochemical, chemical, aerospace, fossil fuel and nuclear power industries, and others.

There are millions of bolts in critical facilities of cited industries, and the problem of structural durability and plant leakage reduction is very complex and involves many areas of applied mechanics and technological findings.

Safe design of bolted flanged connections from structural durability point of view has been, on the whole, solved and standardized, but the plant leakage remains a principal cause of bolt damages and failure that are attributed to the high level of corrosion which is combined with high level of stresses and deformations due to alternating conditions of internal pressure, external loading, elevated temperatures, flow-induced vibrations, integral flow of neutrons, and other critical factors. Hence, the leak tightness has a greater influence on the service life of the bolted flanged connections, and highest priority in plant reliability programs is to limit or to exclude the early leakage, thus protecting critical engineering facilities from untimely degradation and failure.

Statistic data show that, for example, piping system leakages conservatively cost each process industry hundreds of millions of dollars annually in lost profits as a result of plant shutdowns, production penalties, maintenance rework activities, and equipment repair or replacement.

Early plant leakage is closely connected with operating creep of the fasteners and gaskets. Generally, creep is accompanying by stress relaxation, and elongation of bolted fasteners along with contraction of gaskets due to creep-relaxation is a very serious problem because it leads to bolt load and gasket compression losses that, in turn, increase the leakage rate.

It is found that creep-relaxation of bolted fasteners and gaskets is increased with elevated temperature and load-induced stresses, although room temperature creep-relaxation can be also significant even at relatively light loads. A plant maintenance practice includes periodical retightening or replacement of the bolts and gaskets subjected to creep-relaxation to prevent leakages, and, having in mind the great quantity of bolts used in process industries, the procedure involves an expensive time-consuming process and provides only temporary effect of leak tightness because the creep-relaxation increases gradually after additional retightening, and risk of leakage event relatively increases. Nevertheless, during the development of must design procedure, little consideration has been given to the creep-relaxation of bolted fasteners and gaskets, and similar situation is observed with patent documents.

U.S. Pat. No. 6,199,453 to Steinbock, entitled “High temperature bolting system”, offers a sophisticated apparatus for maintaining a clamping force between component parts of a steam turbine while operating at a temperatures of 800 DEG. F to 1200 DEG. F. However, the disclosed elongated stepped fastener shank manufactured from superalloy Inconel 718, having a thermal expansion coefficient similar to flange material and creep strength which is several times greater than creep strength of flanged material, can not stop a creep-relaxation process and protect proposed bolting system from creep-relaxation that is an increase of elongation and decrease of stress with time. Moreover, the high level of stresses and operating temperatures induce the high level of creep-relaxation of the fasteners.

EP Pat. No. 352608 discloses a method of fabrication of reinforced polytetrafluoro-ethylene (PTFE) gasketing materials “characterized by high strength, excellent recovery and superior creep-relaxation resistance”. However, these super characteristics did not exclude the creep-relaxation from 20% to 30% depending on thicknesses of proposed tested materials. Moreover, the tests were carried out under standard procedure during only 22 hours at only 212 DEG. F. Thus, the proposed gasketing materials demonstrate “superior” physical and functional properties when compared to previous PTFE gasketing materials described in prior art. The most important failing, however, is the fact that proposed gasketing materials copy a typically used approach to the fabrication of sealing elements based on traditional “passive” behavior under operating conditions of all known today gasketing materials excepting those described in U.S. Pat. No. 5,226,683 to Julien et al. and U.S. Pat. No. 6,435,519 to White. These two patent documents are the first attempts to introduce the new gasketing material fabricated from NiTi (Nitinol) shape memory alloy.

U.S. Pat. No. 5,226,683 discloses a method to use a gasket of Nitinol shape memory alloy under martensite state to fill the space between the hard flange faces having microscopic surface irregularities that can prevent the fluid leakage between the faces und will allow further to reuse the gasket.

The Nitinol shape memory alloy of which the gasket of this invention is made “remembers” the shape, which it had when it was last formed in its austenite state. When this gasket is deformed under temperature of martensite state it fills the irregularities of flange faces under pressure exerted by hard clamping members of the flanges. The shape memory effect is used when gasket resumes its original shape after heating to austenite state during the restoration step before reuse. Although this invention has failed a main problem of plant leakage reduction by means of creep limitation, it remains a turning point from leakage problem point of view.

U.S. Pat. No. 6,435,519 represents a next attempt to use a Nitinol shape memory alloy as a gasketing material to provide a seal between component parts of an imaginary generalized assembly. Unfortunately, this invention claims a well-known long time procedure to clamp the gasket between adjacent flange faces. As for application of gasket of shape memory alloy, this invention claims the spring forces generated by bending of the gasket when it is in super-elastic state. It is easily to observe that shape memory alloy in super-elastic state displays all mechanical properties of typical elastic material including the property of creep-relaxation while subjecting to elevated temperature and external loading. Meanwhile, this invention tries to open a real way to the application of shape memory alloys as sealing materials even though the problem of creep-relaxation remains out of consideration.

A wide range of patent documents having relation to the present invention is dedicated to the application of shape memory alloys, mostly of the Nitinol, in design of couplings, fasteners, washers, plugs, springs and other structural components for such things as pipes, oil well casings and similar shell structures.

U.S. Pat. Nos. 3,759,552, 4,001,928, 4,149,911, 4,198,081, 4,281,841, 4,450,616, 4,469,357, 4,501,058, 4,537,406, 5,791,847 as well as GB Pat. Nos. 1554432, 1580036, SU Pat. No. 1086282 and JP Pat. No. 62-116292 describe the means of stressing a structural members of Nitinol shape memory alloy components that provide stiffness to shell structures and tubular members as well as prestressed loadings for head bolts or other prestressed fasteners. These documents, however, did not touch the problem of creep-relaxation of mechanical fasteners and gasketing joints.

Japanese Pat. No. 62-188764 describes a method to manufacture a bolt of Nitinol shape memory alloy that may be easily fastened and detached. This bolt is subjected to axial compression and to aging treatment under specific temperatures while holding it under compressive strain. Thus-obtained shape memory alloy bolt reversibly repeats the elongation in a length direction at a temperature of an initial temperature of martensite transformation and the contraction at a temperature of an initial temperature of inverse transformation. Owing to these characteristics, the length of bolt is arbitrary changed, so that bolt may be firmly fastened or easily detached.

The procedure of described bolt production relates to the method known as constrained formation of shape memory effect by means of fixed deformation and following aging treatment under specific temperature above of temperature of austenite state. This procedure is very complex and the bolt's compression under elevated temperature increases a risk of bolt's buckling, and, moreover, the problem of creep-relaxation is failed too.

None of the above-mentioned prior patent documents touch the problem of creep-relaxation of mechanical fasteners and gasketing joints from active intervention to the plant leakage reduction point of view. Accordingly, it is an object of the present invention to form a new approach to provide a bolt-flange-gasket assembly wherein there is a significant limitation or exclusion of creep-relaxation due to operating elevated temperatures and external and/or internal loadings. This invention is the first to introduce a new technological philosophy based on “active” resistance of the fasteners and gaskets to the creep-relaxation under critical operating conditions.

SUMMARY OF THE INVENTION

It is, therefore, a primary object of the present invention to form a new idea of sealing technology based on active behavior of the fasteners and gaskets to limit or to exclude their operating creep while subjecting to the operating elevated temperature and external and/or internal loadings.

It is another object of the present invention to provide a method and devices to limit a creep-relaxation of the fasteners and/or gaskets that are a power elements manufactured from shape memory alloys and previously shape-memorized under temperatures of martensite state.

Active behavior of the power elements results from shape-recovering forces that appear during a recovery of previously shape-memorized deformations under operating elevated temperatures. These forces have a direction inverse to the direction of operating creep deformations of the fasteners and/or gaskets that limits or excludes the elongation of the fasteners and contraction of the gaskets due to creep. This active behavior may be called “negative creep”.

The method consists in application of power elements such as bolts, screws, washers, gaskets, and the like manufactured from shape memory alloys and previously shape-memorized either to tension or to compression, flexion, torsion, or to their combinations. The shape-memorized deformations are obtained during loading-unloading of the power elements under temperature of martensite state when the power elements conserve the residual shape-memorized deformations upon unloading.

The recovery of conserved shape-memorized deformations is occurred after standard tightening of the fasteners and clamping of the gasket during the operating external and/or internal loadings and variety of elevated operating temperatures including the temperatures of martensitic phase transformation of shape memory alloy. The recovery of conserved shape-memorized deformations is further stopped by rigid component parts of the assembly forming an effect of constrained recovery event, so that reactive shape-recovering forces are generated that leads to the stress of fasteners and/or gaskets at the direction inverse to the direction of creep deformations.

The shape memory alloys on a basis of Cu, Fe, Al, Ma, Ga, Ni, Ti, In, Pd, Hf, and others have a large temperature interval of martensitic phase transformations that corresponds to the temperatures to recover the conserved shape-memorized deformations. Hence, the operating temperatures of the assembly have to be in temperature interval of martensitic phase transformations of the power elements manufactured from suitable shape memory alloy. For example, the operating temperatures of Fossil Fuel and Nuclear Power Plants' equipment such as heat exchangers, piping systems, steam generators, coolant system installations, and others vary from dozens to hundreds degrees and remain stable enough for given type of equipment, so that temperatures of martensitic phase transformation of suitable shape memory alloy have to be in the interval of operating temperatures of the assembly. The reactive shape-recovering forces may be considerable depending on quantity of conserved shape-memorized deformations and rigidity of opposed component parts of the assembly.

A most important advantage of the present invention is a new progressive approach to the sealing technology based on active intervention in operating process by means of fasteners and gaskets manufactured from shape memory alloys and previously shape-memorized either to tension or to compression, flexion, torsion, or to their combinations.

Another advantage of the present invention consists in use of reactive shape-recovering forces generated by fasteners and/or gaskets under operating conditions to provide a “negative creep” and to limit or to exclude the creep-relaxation of the fasteners and gaskets.

It is next advantage of the present invention to provide a continuous automatic contact between flange faces and gasket under operating conditions. Reactive shape-recovering forces having direction inverse to the direction of the creep deformations produce this contact.

Proposed devices provide a limitation of creep deformations excluding simultaneously a retightening of the fasteners that increases significantly a leak tightness of critical facilities and extends their service life.

Further brief description of applied drawings and following detailed description of the invention are intended to provide a basis for understanding the nature and character of the present invention, and to explain the principles and operation of presented devices.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 represents schematically a physical basis of constrained recovery when a conserved residual shape-memorized deformation ε_(c) under temperature of martensite state below M_(f) tries to be recovered during the heating to temperature A_(f) of austenite state with appearance of reactive shape-recovering stress σ_(r).

FIG. 2 is a bolted fastener of two component parts of a pressure vessel having a top, base, bolts with nuts and gasket.

FIG. 3 is a similar to FIG. 2 except that the bolts have an internal axial hollow channel adapted to place and to fix firmly a shank manufactured from shape memory alloy and previously shape memorized to the compression.

DETAILED DESCRIPTION OF THE INVENTION—FIG. 3 IS PREFERRED EMBODIMENT

FIG. 1 is three-dimensional stress-strain-temperature diagram σ-ε-T showing a behavior of power element of shape memory alloy during the formation of shape-memorized deformation along with its constrained recovery. An initial point “O” corresponds to temperature below M_(f) at which the transformation of martensite finishes. The stress-induced martensite is obtained during loading-unloading of the power element that is presented at the stress-strain diagram σ-ε at the rear graph in FIG. 1. The point K corresponds to the final stress σ_(f) before unloading, and the point S corresponds to the final strain ε_(c) after unloading.

The strain ε_(C) is a conserved residual shape-memorized deformation of the power element of shape memory alloy. This deformation tries to be recovered during the heating to the temperature A_(f) (point Q at the front graph in FIG. 1 corresponding to austenite state of the power element), but its shape recovery is stopped by rigid component parts of the assembly providing an effect of constrained recovery (point R at the front graph). The constrained recovery of the shape-memorized deformation generates reactive shape-recovering stress σ_(r) that is shown at the front graph in FIG. 1.

A direction of the shape-memorized deformation coincides with direction of operating creep deformation of the fastener or gasket, and, therefore, previously compressed gasket will be shape-memorized to the tension because it will try to recover its initial elongated shape during the heating that will stopped by rigid component parts of the assembly. This constrained recovery of the gasket generates the reactive shape-recovering forces of tension having direction inverse to the direction of operating creep deformation of the gasket. This effect of “negative creep” results in active resistance of the mechanical fasteners to their operating creep that determines a new approach to the sealing technology.

The “negative creep” provides continuous automatic contact between adjacent component parts of the assembly, because, for example, gasket from shape memory alloy being previously shape-memorized to tension will restore its initial elongated shape during the operating process under operating temperature corresponding to the temperature of martensitic phase transformation of applied shape memory alloy. It will be “active” behavior of the gasket, which will actively resist to its operating creep.

The described process of forming of shape-memorized deformation and constrained recovery may be obtained either for tension or compression, flexion, torsion, or for their combinations.

FIG. 2 is a part of the cross section of a pressure vessel having a top 111 and base 12 with flange rings 13 and 14 connected with bolts 15 and nuts 16, compliant gasket 17 manufactured from typically used material being placed between adjacent flange faces. The assembly is subjected to internal pressure “P” and operating temperature “T”.

The bolts 15 are manufactured from shape memory alloy and previously shape-memorized to the compression under temperature of martensite state with suitable quantity of conserved residual shape-memorized deformation. The temperature of martensitic phase transformation of the shape memory alloy corresponds to operating temperature of the assembly, so that previously elongated bolts try to recover their initial length. The recovery process is stopped by resistance of rigid component parts of the assembly with simultaneous generation of reactive shape-recovering forces of compression having direction inverse to the direction of operating creep of the bolts. The reactive forces block a development of bolt elongation due to creep that excludes the bolt retightening and increases the leak tightness of bolted flanged connection.

The same FIG. 2 shows second embodiment of the present invention except the materials of the bolts and gaskets. The bolts of second embodiment are manufactured from typically used structural steel, and the gasket is manufactured from shape memory alloy and previously shape-memorized to the tension with suitable quantity of conserved residual shape-memorized deformation. The process of generation of reactive shape-recovering forces of tension of the gasket is similar to described above, because the gasket tries to recover its initial thickness and, therefore, provides a continuous contact with adjacent flange faces. The reactive forces of the gasket have a direction inverse to the direction of operating creep of the gasket that limits creep development.

The same FIG. 2 relates to third embodiment of the present invention except the materials of the bolts and gasket which are both manufactured from shape memory alloy and previously shape-memorized to the compression (bolts) and to the tension (gasket). Further process is similar to the one described above.

FIG. 3 shows the same component parts of the assembly described in FIG. 2 except that the bolts are manufactured from typically used structural steel and have an internal axial hollow channel adapted to place and to fix firmly a shank 20 manufactured from shape memory alloy and previously shape-memorized to the compression. The gasket may be manufactured from shape memory alloy or from typically used material. Further process is similar to the one described above.

CONCLUSION

The present invention opens a new approach to the sealing technology based on active intervention of the fasteners and gaskets in operating process by means of creep limitation and plant leakage reduction. The power elements manufactured from shape memory alloys may be previously shape-memorized either to tension or to compression, flexion, torsion, or to their combinations that significantly increases applicability of presented method using the feature of shape memory alloys to provide a “negative creep” to limit or to exclude operating creep of the fasteners and gaskets. The real application of the present invention in plant process industries will allow to protect the fasteners and component parts of technological equipment from damages due to the corrosion, and to extend its service life.

It will be understood that various shape memory alloys may by used for manufacturing the power elements to limit the creep of mechanical fasteners and gaskets having temperatures of martensitic phase transformation corresponding to operating temperatures of technological equipment, and various changes in details, materials, and arrangements of the parts of the fasteners and gaskets which have been described and illustrated above in order to explain the nature of this invention may be made by those skilled in the art without departing from the basic principles of the present method as recited in the applied claims. 

1. A method to provide an active behavior of mechanical fasteners to limit or to exclude their operating creep due to external and/or internal loadings and elevated operating temperature of industrial equipment.
 2. A method according to claim 1 wherein said mechanical fasteners are the bolts, gaskets, screws, washers or other fastening power elements.
 3. A method according to claim 2 wherein said bolts, gaskets or other fastening power elements are manufactured from shape memory alloys having temperature interval of martensitic phase transformations corresponding to interval of operating temperatures of industrial equipment.
 4. A method according to claim 3 wherein said bolts, gaskets or other fastening power elements are previously shape-memorized either to the tension or to compression, flexion, torsion, or to their combinations obtaining conserved residual shape-memorized deformations under temperatures of martensite state during loading-unloading.
 5. A method according to claim 1 wherein said mechanical fasteners interact with adjacent rigid component parts of said industrial equipment under temperatures of martensitic phase transformation to provide a constrained recovery of conserved residual shape-memorized deformation that generates a reactive shape-recovering forces having direction inverse to the direction of operating creep of said mechanical fasteners.
 6. A method according to claim 1 that is called the “Method of “negative creep”.
 7. A device to limit or to exclude operating creep of mechanical fasteners including bolts manufactured from shape memory alloy and previously shape-memorized to the compression.
 8. A device according to claim 7 including the gasket manufactured from shape memory alloy and previously shape-memorized to the tension.
 9. A device according to claim 7 including the gasket manufactured from typically used material.
 10. A device including the bolts manufactured from typically used structural steel and gasket manufactured from shape memory alloy and previously shape-memorized to the tension.
 11. A device according to claim 10 wherein the bolts have internal axial hollow channel adapted to place and to fix firmly a shank manufactured from shape memory alloy and previously shape-memorized to the compression.
 12. A device according to claim 11 wherein the gasket is manufactured from typically used material. 